Nat. Biotechnol. 27, 633–641 (2009); published online 28 June 2009; corrected after print 9 September 2009

In the version of this article initially published, the following acknowledgment was inadvertently left out: “The UCSF CPTAC team gratefully acknowledges the support of the Canary Foundation for providing funds to purchase a 4000 QTRAP mass spectrometer.” The acknowlegment has been added to the HTML and PDF versions of the article.